1. Field of the Invention
The present invention is directed to the field of systems for detecting the degradation of quality in oil used as a lubricant in internal combustion engines and more specifically to an on-board sensor used in such a system.
2. Description of the Prior Art
Conventionally, the manufacturers of internal combustion engines and vehicles which employ such engines have advised the purchasers of such products to remove and replace the lubricating oil at prescribed use intervals based upon hours or mileage accumulation. In addition, the manufacturers have also advised that more frequent replacement of the oil may be necessary when the engines are used under severe loading or environmental conditions.
In general, the oil used for lubricating an internal combustion engine should be changed before it loses its ability to prevent engine component wear. Three types of wear mechanisms have been identified as occurring when engine oil is degraded. The first wear mechanism is defined as corrosion caused by chemical attack. The second wear mechanism is erosion which is caused by metal-to-metal contact. The third wear mechanism is abrasion, caused by particles in the oil.
Corrosion does not occur if the oil contains sufficient additives, such as surface protecting compounds like zinc dithiophosphate and detergents, and antioxidants and alkalies to neutralize the corrosive contaminants formed by combustion. However, as the oil ages, these additives are depleted and contaminants such as hydroperoxides and acids, buildup in the oil. Corrosion begins when the peroxides and acids react with the metal surfaces.
Erosion does not occur if a sufficiently thick oil film remains between moving surfaces. The film thickness is controlled by its viscosity. If the oil is too viscous to be pumped throughout the engine, the film disappears where no oil is available. If the oil is too fluid, it is squeezed from between the bearing surfaces and metal-to-metal contact occurs. As fresh oil ages, its viscosity decreases at first due to the destruction of oil thickening additives; later the viscosity increases as oxidation causes polymerization. Erosion can occur if either mechanism causes the oil's viscosity to exceed safe limits. In addition, wear can occur when the antiwear compounds, such as zinc dithiophosphate, are depleted.
Abrasion is due to the accumulation of insoluble particles in the oil, such as airborne dust, carbon, degradation products from the oil and fuel, engine wear debris and corrosion products. Usually the oil suspends these particles and carries them to the oil filter. Fully-formulated oils contain detergents to help suspend these contaminants. However, when the oil loses its dispersive ability through aging, these particles are deposited throughout the engine, and rapid wear begins.
All three mechanisms appear to cause wear in modern engines, but it is usually very difficult to determine if one is responsible for more wear than the others. Indeed, many additives inhibit two wear mechanisms. When they are depleted, both wear mechanisms begin. For example, zinc dithiophosphate and its products adsorb on surfaces protecting them from corrosion and erosion. It also prevents corrosion by removing peroxides from the oil. Another example is detergents which protect surfaces from corrosion and suspend insoluble particles to reduce abrasion.
In general, two separate sensor techniques have been used to measure oil performance: analysis of engine wear and analysis of oil. Assuming that sensors to measure wear and oil are equally feasible to build, monitoring oil appears more desirable than monitoring wear. First, by monitoring engine wear one guarantees that wear of one or more components must begin be#ore the oil change indication is given. One would rather avoid wear, which, perhaps, could be done by making the sensor very sensitive, but this becomes increasingly difficult. Second, wear can occur in many different locations in the engine, such as at the cam shaft, followers, cam shaft bearings, rings, cylinders, main bearings,etc. Each would need a sensor, unless wear in all areas were correlated, which seems doubtful. On the other hand, the oil is well mixed so that measurement or analysis of oil properties in one location is sufficient.
Oil analysis techniques conventionally either determine the depletion of additives or the buildup of contaminants. Additives improve oil by enhancing important functions, such as antioxidation, viscosity, antiwear, anticorrosion and detergency, but in doing so they may undergo chemical reactions. Often intermediate reaction products still perform the original functions, but the final products do not. Unfortunately, laboratory analytical techniques are needed to measure the additives and their reaction products. Sensors that are sufficiently inexpensive and rugged enough for on-board use have not yet been developed, so that the analysis techniques can be used on a vehicle. Even, if such sensors were developed, each active compound must be analyzed, which may possibly require several sensors. Therefore, since it is considered impractical to provide a system that will determine oil quality on-board by analyzing the used oil additive concentration, one is forced confine measurements to the contamination of the oil.
Tests for the buildup of contaminants such as metals, insolubles, acidity and alkalinity have been established. Each test has been correlated to oil or engine performance. For example, insolubles are often correlated to engine cleanliness and wear, total acid numbers are also correlated to wear, and total base numbers to rust and varnish. Viscosity increase is often used as a indicator of oil oxidation. Silicon, which is aspirated into the engine as sand, is commonly correlated to wear.
The concentration of iron clearly results from wear of the iron parts of the engine. Much of the iron remains soluble in the oil as the salts of organic acids. Concentrations of 20 to 100 ppm iron are typical in oil used for 7,500 miles. Iron particles may be present in the oil when rapid wear is occurring, but they should be trapped by the associated oil filter. Copper concentration results from bearing wear when lead and copper bearings are used. Again, much of the copper remains soluble. Lead burned in the fuel can collect in the oil and thereby mask the lead-to-bearing wear correlation.
Results of contaminant tests generally show that large displacement engines cause a higher concentration of oil contamination than small displacement engines. This can be explained for oil oxidation degradation products in that they result from oxidation of the thin film of oil present on the cylinder wall. Large engines have more surface area in the cylinders than small engines so the rate of oxidation is higher. This affect is only somewhat offset by the fact that large engines also have more oil and rotate more slowly than small engines. Overall, the result may be a 20% slower decay of antioxidants in a 2.5 L engine compared to a 5.0 L engine.
Generally, on-board oil change indicators for automotive vehicles are based upon indirect measurements of engine oil condition by monitoring other parameters of the engine.
For instance, U.S. Pat. No. 4,533,900 describes a system which measures the distance traveled by the motor vehicle and, utilizing a preset service interval, modifies the interval to an earlier time based upon historical operating parameters such as engine speed, coolant temperature, oil temperature and/or fuel consumption rate on a weighted basis.
Similarly, U.S. Pat. No. 4,506,337 simulates engine oil wear by sensing the number of engine revolutions per time and the load on the engine. Those factors are used in calculating the amount of soot suspended in the lubricating oil.
U.S. Pat. No. 4,497,200 and 4,525,782 also describe similar systems for simulating or estimating oil degradation.